Westminster attacker acted alone:UK police

British police investigating an assault on Parliament that claimed four victims and wounded dozens say they still believe the assailant acted alone.

Police also say there is no information indicating that further attacks are planned.

Deputy Assistant Commissioner Neil Basu said it may never be possible to fully determine the motives of attacker Khalid Masood, who was shot dead pm Wednesday after running over pedestrians with an SUV and fatally stabbing a policeman.

An unknown family pose with PC Keith Palmer. ()

"That understanding may have died with him," Basu said as police appealed for people who knew Masood or saw him to contact investigators.

"Even if he acted alone in the preparation, we need to establish with absolute clarity why he did these unspeakable acts, to bring reassurance to Londoners."

The Islamic State group has claimed Masood was a "soldier" carrying out its wishes for supporters to attack Western countries.

Police raced to the scene of the terror attack on Wednesday. ()

One man remains in custody in the case. He has not been charged or named. Nine people arrested after the assault have been released without being charged and one has been freed on bail.

The family of slain police officer Keith Palmer, meanwhile, released a statement thanking those who tried to save his life.

"There was nothing more you could have done. You did your best and we are just grateful he was not alone," the statement said.

Five people died in the London terror attacks. ()Mourners gathered to lay tributes on Westminster Bridge. ()

Masood, a 52-year-old Briton, had convictions for violent crimes and spent time in prison. He also worked in Saudi Arabia teaching English for two years and travelled there again in 2015, apparently on a religious pilgrimage.